The notion of Quality of service, QoS, is used in cellular systems such as long term evolution, LTE, in order to assist one or more scheduling functions in the system to prioritize between users and services.
As specified in 3GPP (see for example specification 3GPP 23.203), QoS class indicators, QCIs, are used to classify different services in order to achieve the QoS concept. For example, QCI1 is specified as Conversational Voice and QCI2 is defined as Conversational Video (Live Streaming) in 3GPP 23.203. In LTE systems, in order to add the QoS concept to the LTE scheduling algorithm, priority has been introduced in order to prioritize between difference QCIs.
Following the QoS concept, a service with higher priority should be scheduled first, compared to a service with lower priority, and thus, in a scheduler, the priority will to a large extent be interpreted as a scheduling weight.
Users with a higher scheduling weight will be scheduled before user with lower weight. However, usually, a scheduler will not only consider the QCI priority, but also the channel quality or the accumulated delay of a packet to be scheduled, according to specific algorithms in the scheduler.
In general, according to the concept of QoS, the priority of QCIs gives a large impact on the scheduling “weight”. Therefore, in a system which is highly “loaded” with high priority user equipments, UEs, a UE with low priority might not be able to be scheduled at all, and will thus run the risk of being “starved”, due to the lower scheduling weight caused by the UE's low priority.
Different priorities between different UEs may also cause a problem related to the starvation, in cases where the so called Radio Link Control Acknowledged Mode, RLC AM, is used, as explained in the following:
Radio link control, RLC, protocol is one of the L2 protocols used in the LTE air interface, Uu, which connects an UE with “its” eNB. There are three transmission modes, TMs, used for RLC transmission protocol:                RLC transparent mode, TM,        RLC unacknowledged mode, UM,        RLC acknowledged mode, AM.        
RLC AM transmissions from a transmitter relies on receiving RLC status reports, e.g. ACK/NACKs, from the intended receiver in order to see if a transmitted packet has been received successfully or not. Packets which are not received or which are received incompletely will be scheduled for retransmission later. However, if an RLC status report is not received after a certain period of timed, a retransmission of the packet will be scheduled automatically. After several retransmissions without received status reports, the RLC connection will be seen as a failure, and the UE is “dropped”, as specified in 3GPP 36.322.
The traffic in downlink, DL, and uplink, UL, are not fully aligned with each other, i.e. there will be cases or situations in which there is a low traffic load in one direction, DL or UL, and a high traffic load in the other direction, UL or DL. In such cases, RLC AM traffic from low-priority UEs will be transmitted in the direction with the lower load, but the required corresponding RLC status report (ACK/NACKs) will be starved by higher priority UEs in the other direction, which will result in RLC failure, and the low priority UE will be dropped due to this. It can be mentioned here that the priority of a UE is the same for data and for the UE's RLC status reports, i.e. if the data traffic to/from a UE has low priority, the corresponding RLC status reports from/to the UE will have the same priority, which will lead to the possibility of a UE being dropped due to “starved” RLC status reports.
It can be added that the scheduling of traffic to (“downlink”) and from (“uplink”) the UEs in a cell is usually performed by a scheduler in the controlling node of the cell, the controlling node in LTE systems being referred to as the eNodeB or eNB.